The personal tragedy and societal burden of Alzheimer's disease are steadily mounting, and an approved disease-modifying therapy still lies years away. There is widespread agreement in the AD field that slowing and ultimately halting this complex, multi-cellular syndrome may require secondary, and ultimately primary, prevention, but this means we must understand the initiation of disease much more clearly and must have facile biomarkers to detect and follow it in its beginning stages. Many lines of evidence support an early, perhaps initiating role for the accumulation of soluble forms of amyloid ?-protein, but the identity of the most bioactive forms in human brain remains murky. This grant provided early evidence for the importance of soluble A? oligomers, but many unsolved questions about them remain. Here, we will use our lab's extensive biochemical experience in studying the most AD-relevant forms of A? ? soluble oligomers isolated directly from AD cortex ? to conduct novel experiments that address mechanistic, therapeutic and diagnostic obstacles to clinical success. In this carefully revised A1 application that has tried to respond to all of the concerns of the Study Section, we will pursue this goal by systematically carrying out three independent but related aims: Aim 1: To explore the response of endogenous microglia in vivo to human brain A? oligomers (oA?) administered icv to healthy aged mice in standard vs. novelty-rich environments (EE). We will use quantitative in situ microglial morphometry combined with unbiased mRNA screens of FACS-purified microglia to quantify many inflammatory genes and learn which signaling pathways are responsible for mediating the potent prevention by EE of the adverse in vivo microglial responses to oA? we have recently discovered. Aim 2: To extend our striking new biochemical evidence that ?not all oligomers are created equal?: typical AD brains yield high levels of high MW A? oligomers that don't alter synapses and microglia, whereas smaller oligomers derived from them have potent cytotoxicity. Here, we will use new oA?-specific ELISAs we've designed and novel biological readouts to quantify the `specific activities' of different oligomers isolated from typical late- onset AD brains. We will then define the ability of various anti-A? monoclonals, including some in human trials, to neutralize the most biologically active AD brain oligomers. Aim 3: To approach a major unmet need: the validation of a sensitive, specific and inexpensive blood test for AD that could ultimately be useful in everyday practice. Using ultra-sensitive instruments (the Simoa and Erenna platforms), we will quantify several candidate blood markers: plasma tau, oA?, A?42 monomers, ApoA-II, cortisol, and others. Then, we will probe CNS-derived exosomes in human blood that contain A?42, tau and other neural analytes. We will monitor changes in the ratios in plasma and exosomes of certain analytes (e.g., p-Tau/A?42) assessed longitudinally in pre-symptomatic and early symptomatic AD subjects. Collectively, our new results should illuminate very early events in AD pathogenesis, with attendant diagnostic and therapeutic implications.